1. Field of the Invention
The present invention relates generally to a device for reproducing magnetically recorded data and more specifically is directed to an apparatus for reproducing video signals which have been digitized.
2. Description of the Prior Art
In currently used types of video recording apparatus, the video signal is frequency modulated and then recorded as an analog signal on magnetic tape.
However, in the case where the video signal is converted into a digital signal prior to recording, it is possible to repeatedly dub or copy the recorded signals without incurring any deterioration in the characteristics thereof.
As shown in FIG. 4, when a signal is reproduced from a magnetic tape, the magnetic head, the tape and the like of the electromagnetic transducing system exhibit characteristics which, in the low frequency and high frequency regions, cause reduction in the S/N ratio.
Accordingly, depending on the reproducing system, in order to obtain the desired S/N value, a digitized video signal (hereinafter referred to as a digital signal) may be limited to a narrow frequency range. Therefore, it is necessary to use a recording system by which the frequency spectrum enables the S/N ratio to approach its maximum value.
In order to achieve the above, a method has been proposed wherein the digital signal is recorded by means of a single detection class IV type partial response technique. In other words, as the S/N ratio deteriorates at high and low frequencies, it is possible to use a delay operator D value in the form of a class IV partial response (1-D.sup.2) in a manner to proximate the frequency characteristics H(.omega.), as shown on FIG. 5.
A video recorder, which has been proposed in order to take advantage of the fact that digital video signals can be dubbed without undergoing any deterioration, is shown on FIG. 19. In this arrangement, predetermined recording data D.sub.REC is supplied via an amplifier 3 to a magnetic recording head 4 and recorded thereby on magnetic tape 5. During reproduction, a magnetic playback head 6 is used to reproduce a signal S.sub.RF which is supplied through an amplifier 8 and an equalizing circuit 9 to a decoding circuit 10.
A clock signal Sck is produced by a clock signal generator 12 and supplied to the decoding circuit 10. The decoding circuit 10 responds thereto by checking the level of the reproduced signal S.sub.RF with respect to a predetermined reference value, at a predetermined timing, and outputting reproduction characteristic data D.sub.RB which is then used to decode the digitized video signal.
However, with this arrangement as a result of the movement of the tape and the inherent characteristics of the reproduction system, fluctuations in the level of the signal S.sub.RF cannot be avoided.
If an automatic compensation circuit is provided to correct the level of the signal S.sub.RF, a slice level value in the decoding circuit 10 may become variable and it then becomes necessary to prevent bit error from appearing in the reproduction characteristic data.
However, with the above described method, in the event that noise is contained in the reproduced signal, bit error inevitably occurs.